This invention relates to a drive circuit for exciting the transducer of an ultrasonic level gauge, and more particularly to a drive circuit adapted to generate AC drive power that rises quickly at the start of transmission and falls at the end thereof.
In an ultrasonic level gauge, the distance extending between a measuring station and an object, such as the surface of a liquid, is measured by determining the elapsed transit time from the emission of an ultrasonic wave at the station and the reception thereat of the reflection of the wave from the surface of the liquid or object to be measured. It is important that the emitted ultrasonic wave rise quickly at the start of transmission and fall rapidly at the end thereof.
Effective reception of the reflected wave depends on whether the amplitude level of the reflected wave attains a reference level which is higher to a predetermined degree than the noise level. But with a conventional drive circuit which makes use of an AC oscillator having an L-C resonant circuit, the emitted wave does not rise sharply at the start of emission, nor does the reflected wave rise sharply at its beginning. Consequently, a measurement error is encountered, for variations occur in the time it takes for the reflected wave to attain the predetermined reference level.
Furthermore, when the emitted ultrasonic wave does not drop sharply at the end of transmission, it becomes difficult to measure short distances. The reason for this difficulty is that the vibrations of the ultrasonic transducer are not damped with sufficient rapidity. Also, the reflected wave and the transient at the termination of emitted ultrasonic wave are in overlapping relationship.
To overcome this rise-fall time problem, one must improve the drive circuit for exciting the ultrasonic transducer. For this purpose, it is desirable to render constant the frequency and the amplitude of the AC excitation power within the excitation period. It is also desirable to equalize the total amount of positive half waves and the total amount of negative half waves constituting the AC excitation power, so as to terminate with sufficient rapidity the vibrations of the ultrasonic transducer at the measuring station.
But with a conventional drive circuit using an AC oscillator having an L-C circuit, it takes some time for the amplitude and for the frequency of the oscillator output to attain a constant state; hence the rise time characteristic of the AC excitation power does not fulfill the requirements for an ultrasonic level gauge. Furthermore, with a conventional drive circuit, a large transient occurs at the end of the excitation.